1. Field of the Invention
The present invention relates generally to the field of memory devices and more particularly, in one or more embodiments, to trimming internal voltages of flash memory.
2. Description of the Related Art
Flash memory is a non-volatile memory that can be electrically erased and reprogrammed. It is primarily used in memory cards, USB flash drives, and the like for storage of data in computer systems. Generally, flash memory stores information on an array of floating gate transistors, called “cells”, each of which traditionally stores one bit of information. Each cell is characterized by a threshold voltage (Vt). By adding or removing charge from a floating gate, the threshold voltage of the cell changes, thereby defining whether the cell is programmed or erased. The threshold voltage level indicates the value for a single bit of information, generally represented as a 1 or 0.
Other flash memory devices, referred to as multi-level cell devices (MLC's), can store more than one bit per cell by charging the floating gate to different levels. This technique varies the threshold voltage across a set of ranges. A two bit value is assigned to each specific voltage range, as opposed to a single bit assigned to each range above and below a single value. In a two bit MLC, four voltage ranges may be used to represent the four possible values of the two bits, i.e., 00, 01, 10, and 11.
To increase the number of bits stored on a single cell, MLC devices should be able to manage voltages and charges precisely. For instance, during programming a precise charge must be placed onto the floating gate of a cell to produce a voltage within a desired range. Similarly, during a read operation, a precise read voltage is placed on the control gate, the source is grounded, and a drain bias is applied across the cell, such that the cell conducts a current proportional to the cell's voltage. In a MLC that stores two bits per cell, the voltages supplied may include three program verify voltages, three read voltages, and one erase verify voltage. Accordingly, a memory device may include numerous voltage sources that provide voltages to the cells of the memory array or to other locations within the memory.
The supplied voltages are generally provided by internal voltage generators that are connected to the transistor making up each cell. For instance, each memory die may include multiple voltage generators configured to output a voltage for the program, read and write operations. Often, there may be variations between the desired voltage level output and the actual voltage level output from each voltage source. In memory applications, including MLC applications, it is often desirable that each voltage generator be internally adjusted (e.g., modified, regulated, calibrated, and/or trimmed) to provide an accurate voltage level. Individually adjusting each voltage source of a memory device can be time consuming and costly. Further, as the number of memory devices and the number of supply voltages increase, such as in MLC applications, the number of desired adjustments may increase drastically, and require more time and effort.
Embodiments of the present invention may be directed to one or more of the problems set forth above.